The energy crisis of the 1970's has reemerged as a global issue with an increase in competition for existing oil resources. Rising standards of living, increased industrialization, and a growing worldwide population are driving this competition. Among the various alternative energy strategies, a hydrogen-based energy infrastructure is a particularly promising approach that offers several advantages. First, hydrogen is the third most abundant element and offers the highest energy density per unit weight of any known fuel. The heat of combustion for H2 (125 MJ/kg), for example, is about 300% greater than that of gasoline (43 MJ/kg). Second, hydrogen is the lightest element and would therefore enhance efficiency by reducing fuel storage weight relative to petroleum fuels. Third, hydrogen is also a renewable resource and can be produced from a variety sources, such as steam reforming of natural gas, electrolysis of water, and photosynthesis of CO2, H2O and sunlight to H2 and O2. Fourth, hydrogen is environmentally friendly and may lead to reducing greenhouse gas emissions because water is the byproduct of a hydrogen combustion engine or a hydrogen fuel cell.
One significant challenge of implementing a hydrogen-based energy infrastructure is storing hydrogen in sufficient quantities for transportation and other widespread applications. For example, high pressure storage of hydrogen for such applications is impractical because hydrogen ignites at atmospheric pressures in concentrations as low as about 4% by volume. One alternative to high-pressure storage of H2 is cryogenic liquification. Although liquification of hydrogen clearly has advantages over high-pressure storage, such as a higher energy density and the ability to operate at atmospheric pressure, it is not practical because the energy loss of liquefying hydrogen and prolonged cryogenic cooling can be as high as 40% (Crabtree, et al. Physics Today 57, 39, 2004). Another alternative to high pressure storage is combining hydrogen with metals in the form of metal hydrides. Some light metals, such as magnesium and lithium, react with hydrogen to produce metal hydrides that can later release high purity hydrogen similar to water in a sponge. The total adsorbed hydrogen is generally 1%-2% in gravimetric density (ratio of adsorbed H2 mass to the total mass), and in some cases storage densities as high as 5%-7% have been reported (Chen, et al., Nature 420, 302, 2002; Leng et al., J. Phys. Chem. B 108, 8763, 2004; Pinkerton, et al., J. Phys. Chem. B 109, 6, 2005). Metal hydrides, however, are not practical in many applications because high temperatures (˜300° C.) are needed to achieve sufficient rates of hydrogen release (Crabtree, et al. Physics Today 57, 39, 2004).
Another alternative for storing hydrogen is to adsorb hydrogen onto the surfaces of nanomaterials that facilitate low temperature desorption. Lightweight nanomaterials (e.g. nanotubes, nanohorns, and other row one and row two main group structures) have high surface area to weight ratios that can achieve high storage densities, but as indicated in a recent Department of Energy (DOE) report, the maximum storage of adsorped hydrogen is currently only 4.1% (Department of Energy, Office of Science, Argonne National Laboratory: Basic Research Needs for the Hydrogen Economy, 2003). To meet even the 2010 hydrogen storage target of 6%, multilayer adsorption of H2 is desirable (Department of Energy, Office of Science, Argonne National Laboratory: Basic Research Needs for the Hydrogen Economy, 2003). Several groups have investigated the use of different types of nanostructured carbon, such as activated carbon, nanotubes, and nanofibers, for hydrogen uptake (Dillon, et al., Nature 386, 377, 1997; Züttel, et al., Int. J. Hydrogen Energy 27, 203, 2002; Lamari, et al., Int. J. Hydrogen Energy 27, 193, 2002; Hirscher, et al., J. Alloys Compd. 356-357, 433, 2003). However, carbon nanostructures have several limitations including cryogenic desorption temperatures and low adsorption rates at room temperature. In the case of carbon nanotubes, hydrogen capacity rates have been difficult to reproduce/confirm as noted by the reported gravimetric storage capacities ranging from 0.25% to an unrealistic 20% (Chen, et al., Science 285, 91, 1999; Mpourmpakis, et al., Nano Lett. 6, 1581, 2006; Marella, et al., Carbon 44, 1404 2006).
Oxide ceramics may represent an alternative to carbon-based materials for hydrogen storage applications. It has been suggested in the art that ceramic oxide nanostructures (e.g. nanotubes and other materials with nanoscale structures) may provide alternative compositions for use in storage. However, little data and few examples have been provided to yield sufficient information to predict suitable compositions (Bradley, et al, U.S. Pat. No. 6,672,077). Theoretical studies have indicated that vitreous boron oxide (B2O3) exhibits suitable surface properties for H2 storage (Jhi, et al, Phys. Rev. B 69, 245407, 2005; Jhi, et al., Phys. Rev. B 71, 035408, 2005). Some experimental support exists for the use of boron oxide derivatives to store hydrogen, but these also reportedly have cryogenic desorption temperatures that limit practical utility (Jhi, et al, U.S. Pat. No. 6,991,773). These investigations suggest the possibility for use of boron oxide ceramics in hydrogen storage applications based upon the energetics of the H2—B2O3 interactions, but many practical requirements remain to be solved. Specifically, methods that reliably produce materials which have both a high surface area and high degree of surface accessibility are limitations to the realization of materials with the desired characteristics for hydrogen storage applications. Moreover, it would be desirable to develop a material that induces multilayer adsorption of hydrogen. As a result, there is a need to further develop and explore new material structures and compositions for storing hydrogen.